1. Field of the Invention
The present invention relates to a bump electrode structure and more particularly to a small bump electrode or protruded electrode provided on a semiconductor chip for connecting the integrated circuit device to the outside. The present invention also relates to a semiconductor chip having such a bump electrode structure.
2. Description of the Prior Art
Recent development of high integration techniques for fabricating semiconductor integrated circuit devices has naturally led to requirement for rationalization of forms in which the semiconductor integrated circuit devices are-mounted in electronic circuits or electronic devices, particularly reduction in time and effort, as well as spaces required for mounting. As is well known, it is more advantageous to mount chips of integrated circuit devices as they are, i.e., in the form of chips themselves, than mount them after they are contained in plastic packages or the like. Hence, so-called flip chips, which are mass-produced electronic devices having protruding bump electrodes have been adopted widely.
Referring to FIG. 1, a typical example of construction of a conventional bump electrode will be explained below. FIG. 1 is a cross sectional view showing a portion of a semiconductor integrated circuit device chip. As shown in FIG. 1, a bump electrode 20 made of a solder is mounted on a semiconductor region 10 of a semiconductor integrated circuit device chip 1. An insulator film 22 such as silica film covers a surface of the semiconductor region 10. An aluminum wiring 23 connected to the integrated circuit in the semiconductor region 10 is arranged on the insulation film 22. An end of the aluminum wiring 23 is covered with a protective film 24 made of silicon nitride or the like, which is formed with a window for forming the bump electrode 20 by removing the protective film 24 to expose the aluminum film 23. The exposed aluminum film area or the window area is covered with a thin subbing layer 25 made of a metal such as titanium or copper. Then, a metal protrusion 26 made of a solder is grown to a predetermined height by an electroplating method. In the case shown in FIG. 1, the metal protrusion 26 is heated for a short time to melt the solder, thereby forming the bump electrode 20 rounded on its top.
The mounting of a flip chip having such a bump electrode as mentioned above can be performed, for example, by placing the chip on a device on which the chip is to be mounted in such a manner that the tip of the bump electrode contacts a conductor of the other device, that is, upside down in FIG. 1, and heating in this state to melt the solder to thereby solder the metal protrusion 26 to the conductor. This ensures both fixation and connection of the flip chip to the other device on which it is to be mounted simultaneously, resulting in reduction in the labor of mounting and minimization of spaces required for the mounting.
The development of high integration of circuits on the other hand has led to reduction in size of chips of semiconductor integrated circuit devices and increase in the number of circuits integrated in a single chip, and hence there is a tendency that the number of connections connected to the outside increases. In the case of certain integrated circuit devices, for example, it is required to arrange several hundreds bump electrodes on a surface of a chip of within several millimeter square. This means the size of the bump electrode must be reduced from about 100 .mu.m in diameter or square as had by conventional bump electrodes, to below 30 .mu.m. However, miniaturization or reduction in size causes a new problem of weakened attachment or junction of the bump electrodes to the root of the chip
More specifically, reduction in the diameter of a bump electrode from 100 .mu.m to 30 .mu.m results in reduction in the area of the root portion the bump electrode to about 1/10, and this alone leads to reduction in junction strength by one order of magnitude. In addition, reliability of adhesion among the films provided between the metal protrusion 26 and the semiconductor region 10, i.e., the insulation film 22, aluminum wiring film 23, the protection film 24 and the subbing film 25 shown in FIG. 1 also reduces so that decrease in the junction strength is further aggravated to such an extent that a small outer force urged upon the mounting could readily cause the films to peel one from another.